The present invention relates to an electromagnetic flowmeter which measures the flow rate of a fluid to be measured, which flows through a measuring pipe and, more particularly, to an exciting method and signal processing method capable of realizing accurate flow rate measurement.
An electromagnetic flowmeter measures the flow rate of a conductive fluid to be measured, which flows through a measuring pipe, by converting the flow rate into an electrical signal by using electromagnetic induction. FIG. 11 shows the arrangement of a conventional electromagnetic flowmeter.
This electromagnetic flowmeter comprises a measuring pipe 11 through which a fluid to be measured flows and a pair of electrodes 12a and 12b which oppose each other in the measuring pipe 11 to be perpendicular to both the magnetic field applied to the fluid to be measured and an axis PAX of the measuring pipe 11 and also come into contact with the fluid to be measured. The electrodes 12a and 12b detect an electromotive force generated by the magnetic field and the flow of the fluid to be measured.
The electromagnetic flowmeter also comprises an exciting coil 13 which applies, to the fluid to be measured, a magnetic field perpendicular to both the measuring pipe axis PAX and an electrode axis EAX that connects the electrodes 12a and 12b, a signal conversion section 15 which detects the electromotive force between the electrodes 12a and 12b, and a flow rate output section 16 which calculates the flow rate of the fluid to be measured from the interelectrode electromotive force detected by the signal conversion section 15.
In the electromagnetic flowmeter shown in FIG. 11, a plane PLN which includes the electrodes 12a and 12b and is perpendicular to the direction of the measuring pipe axis PAX is defined as a boundary in the measuring pipe 11. At this time, symmetrical magnetic fields are applied to the fluid to be measured on both sides of the plane PLN, i.e., the boundary in the measuring pipe 11. The exciting coil 13 can be excited by a sine wave exciting method or a rectangular wave exciting method (e.g., xe2x80x9cA to Z of Flow Rate Measurement for Instrumentation Engineersxe2x80x9d edited by Japan Measuring Instruments Federation, Kogyogijustusha, 1995, pp. 143-160).
The sine wave exciting method that uses a sine wave as an exciting current for an exciting coil is readily affected by commercial frequency noise. However, this problem can be solved by a high-frequency exciting method which uses an exciting current having a higher frequency.
The high-frequency exciting method is resistant to 1/f noise such as electrochemical noise or spike noise. In addition, this method can improve the response (a characteristic which makes a flow rate signal quickly follow a change in flow rate).
However, the conventional sine wave exciting method is readily affected by in-phase component noise. An example of in-phase component noise is a shift of the amplitude of a magnetic field applied to a fluid to be measured.
In the conventional electromagnetic flowmeter, when the amplitude of the exciting current supplied to the exciting coil varies (shifts) due to a fluctuation in power supply voltage, and the amplitude of the magnetic field applied to the fluid to be measured shifts, the amplitude of the interelectrode electromotive force changes, resulting in a flow rate measurement error due to the influence of shift. Such in-phase component noise cannot be removed even by the high-frequency exciting method.
To the contrary, the rectangular wave exciting method that uses a rectangular wave as an exciting current to be supplied to an exciting coil is resistant to in-phase component noise.
In the rectangular wave exciting method, however, the interelectrode electromotive force is detected when a change in magnetic field becomes zero. When the exciting current has a high frequency, the detector must have high performance.
Additionally, in the rectangular wave exciting method, when the exciting current has a high frequency, effects of the impedance of the exciting coil, the exciting current response, the magnetic field response, and an overcurrent loss in the core of the exciting coil or measuring pipe cannot be neglected. It is difficult to maintain rectangular wave excitation.
As a result, in the rectangular wave exciting method, high-frequency excitation is difficult, and an increase in response to a change in flow rate or removal of 1/f noise cannot be realized.
It is an object of the present invention to provide an electromagnetic flowmeter which can remove in-phase component noise and correct any flow rate measurement error and also realize high-frequency excitation.
In order to achieve the above object, according to the present invention, there is provided an electromagnetic flowmeter comprising a measuring pipe through which a fluid to be measured flows, an electrode which is arranged in the measuring pipe and detects an electromotive force generated by a magnetic field applied to the fluid and flow of the fluid, a first exciting coil which is arranged separately from a plane, which includes the electrode and is perpendicular to a direction of an axis of the measuring pipe, and applies a first magnetic field having a first frequency to the fluid, a second exciting coil which is arranged on a side opposite to the first exciting coil with respect to the plane and applies, to the fluid, a second magnetic field obtained by amplitude-modulating a carrier having the first frequency by a modulated wave having a second frequency, a power supply section which supplies an exciting current to the first exciting coil and the second exciting coil, a signal conversion section which separates a component of the first frequency from the electromotive force detected by the electrode to obtain an amplitude, separates one of components of sum and difference frequencies of the first and second frequencies from the electromotive force to obtain an amplitude, and obtains a ratio of the amplitudes, and a flow rate output section which calculates a flow rate of the fluid on the basis of the amplitude ratio obtained by the signal conversion section.